Three processes are used, among other things, for preparing the phosphor layer in the production of cathode ray tubes for monitors, display screens, etc.: The brushing process, the spin coating process, and electrophoresis.
The substances of the phosphor layer, hereinafter called phosphors, are applied by means of a brushing sponge in the brushing process. However, phosphor layers applied in this manner have a relatively low homogeneity, Furthermore, the excess material is wasted and must be recycled or disposed of in an expensive process.
In the spin coating process, the phosphor layer is prepared from a mixture of a lacquer paste based on, e.g., ethyl cellulose and the phosphors. The phosphor-lacquer mixture is applied to the rear side of the picture side, i.e., to the side facing a later observer. After the distribution of this lacquer mixture, the cathode ray tube is rotated around an axis orthogonal to the picture side. After drying of the lacquer mixture, the lacquer mixture is heated to above 200.degree. C. and especially up to 400.degree. C., as a result of which the binder is destroyed and the phosphors remain on the rear surface of the picture side. However, the distribution of the phosphors is inhomogeneous, as a result of which the layer thickness of the phosphor layer, especially the layer thickness measured in parallel to the surface of the picture side, displays great variations. This process is unsuitable for coating large surfaces due to these differences in layer thickness.
In electrophoresis, the rear side of the display screen is coated with an electrically conductive electrode layer, mostly one consisting of a metal, such as aluminum or ITO (Indium-Tin Oxide). The phosphors are suspended in a solvent. An electrolyte or a mixture of various electrolytes, which dissolve in the solution and/or are suspended in it, is also added to the solution. The solution or the suspension is charged by the electrolyte. To deposit the phosphors and consequently to form the phosphor layer, the solution is brought into contact with the electrode layer, the electrode layer is used as an electrode, a counterelectrode is introduced, and different electrical potentials are applied to two electrodes, as a result of which the phosphors are electrophoretically deposited on the electrode layer, forming the phosphor layer. The layer thickness of the phosphor layer can be set by varying the parameters electrical (deposition) voltage, temperature, deposition time, and the materials used. The phosphors are subsequently optionally to be fixed on the rearward picture side by heating. After the application of the phosphor layer and the optional fixation, the electrode layer must be removed, especially by chemical etching. However, this process has a high reject rate.